Bug 36651: Add placeholder text to the search bar in the OPAC

Bug 36651: Add placeholder text to the search bar in the OPAC

This patch adds placeholder text to the OPAC search bar, with JavaScript
to change the placeholder based on the user's search type selection.

To test, apply the patch and go to the OPAC.

- The search bar should by default have the placeholder "Search the
  catalog by keyword"
- Change the search type (Author, Title, etc). The placeholder text
  should change accordingly.

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>